1. Field of the Invention
The present invention relates to optical interferometers, and more particularly to an improved Fabry-Perot (F-P) interferometer.
2. Description of the Prior Art
The interferometer has long been used to resolve wavelengths precisely and is still a major tool for spectroscopy. Highest resolution is obtained with the use of multipass (multierflection) interferometers such as the Fabry-Perot (F-P) interferometer which is described in detail in LASERS & OPTRONICS, May 1987, Gordon Publications, Inc., Dover, NJ, pages 57-60. This device uses an etalon having partially-reflecting partially-transmitting mirrors arranged so that light of the desired wavelengths constructively interfere and light of other wavelengths destructively interfere to produce very sharp fringe patterns.
In a single zone F-P interferometer, a mask with a pinhole is placed at the focal point of a lens following the interferometer. The pinhole aperture admits only the set of wavelengths which constructively interfere at the incident angle represented by the pinhole. This severely limits the angle of acceptance of the incident light and has a major negative impact on the etendue or throughput of the device.
A multi-zone F-P interferometer uses a mask, as explained below, in the lens focal plane which admits only light at incident angles associated with the different orders of constructive interference. Since more light is admitted with this device, the etendue is improved in proportion to the number of zones provided. Nevertheless, a trade-off still exists between the resolution and the total acceptance solid angle; a higher finesse [ratio of the distance to the next order constructive interference (fringe) to the width of the fringe] reduces the width of each acceptance angle. The mask takes the form of a series of concentric transparent rings or annuli which are positioned to admit only light at the desired wavelength. Other wavelengths interferring at other angles are blocked by the non-transmitting portion of the mask.
The multi-zone F-P interferometer then accepts only signals within a series of discrete angular rings or zones about the optical axis. Light at the desired wavelengths but at other angles do not pass through the interferometer but are reflected back from it.
This invention is directed to an improved F-P interferometer which recovers these otherwise lost or reflected wavelengths.